Transistor Or Resistor
in [Basics]
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From: John Fields on 1 May 2010 17:10 On Sat, 01 May 2010 08:07:41 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Fri, 30 Apr 2010 18:18:57 -0500, Chris W <1qazse4(a)cox.net> wrote: > >. >. >. > >>In the end I think I will probably end up attaching a wire with >>powerpole connectors to each resistor and powerpole connectors to a >>common buss bar then plug in more and more resistors to change the load. > >--- >That's probably best. > >If you want a simple, less klugie way to do it, though, you might want >to try this: > >1. Determine the maximum current into the load box. >2. Determine what resolution you want out of the load box. > >Let's say you want 50 amps max into the box and you'd like to switch the >load in 5 amp steps. > >Since 50A/5A per step = 10, you know you're going to need 10 resistors >and 10 transistors, and with a little bit of work you can figure out >that if you've got a 12V supply and you want to pull 5 amps out of it, >the load resistance required to do that will be: > > E 12V > R = --- = ----- = 2.4 ohms, > I 5A > >the resistor will dissipate: > > P = IE = 5A * 12V = 60 watts, > > >and the finished circuit will look like this, on the ends, with eight >identical stages in between. > > +-----+ > +-----------------------|+12 | > | R1 Q1 | | > +--[2R4]--D S------+--|GND | > | G | +-----+ > | | | DUT >S1>---|-----------+--[1k]--+ > | | > . . > . . > . . > | | > | | > | | > | R10 Q10 | > +--[2R4]--D S------+ > G | > | | >S10>--------------+--[1k]--+ > >The advantages? > >If you use logic level MOSFETS with an Rds(on) of 20mV at 5A Id, that's >a dissipation of 100 milliwatts per device, which means _no_ heat sink >at all. > >Plus, you'll be able to drive the gates with 5V CMOS if you want/need >to. --- Slow Saturday: Version 4 SHEET 1 880 680 WIRE -992 16 -1152 16 WIRE -832 16 -992 16 WIRE -672 16 -832 16 WIRE -512 16 -672 16 WIRE -352 16 -512 16 WIRE -192 16 -352 16 WIRE -32 16 -192 16 WIRE 128 16 -32 16 WIRE 288 16 128 16 WIRE 448 16 288 16 WIRE -1152 48 -1152 16 WIRE -992 48 -992 16 WIRE -832 48 -832 16 WIRE -672 48 -672 16 WIRE -512 48 -512 16 WIRE -352 48 -352 16 WIRE -192 48 -192 16 WIRE -32 48 -32 16 WIRE 128 48 128 16 WIRE 288 48 288 16 WIRE -1152 160 -1152 128 WIRE -992 160 -992 128 WIRE -832 160 -832 128 WIRE -672 160 -672 128 WIRE -512 160 -512 128 WIRE -352 160 -352 128 WIRE -192 160 -192 128 WIRE -32 160 -32 128 WIRE 128 160 128 128 WIRE 288 160 288 128 WIRE 448 160 448 16 WIRE -1200 304 -1200 240 WIRE -1040 304 -1040 240 WIRE -880 304 -880 240 WIRE -720 304 -720 240 WIRE -560 304 -560 240 WIRE -400 304 -400 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416 288 416 WIRE -1200 496 -1200 416 FLAG -1200 496 0 SYMBOL voltage 448 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL nmos 240 160 R0 SYMATTR InstName M1 SYMATTR Value IRL3915 SYMBOL voltage 240 288 R0 WINDOW 0 -40 3 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 5 0 1e-6 1e-6 1 2) SYMBOL nmos 80 160 R0 SYMATTR InstName M2 SYMATTR Value IRL3915 SYMBOL voltage 80 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value PULSE(0 5 .1 1e-6 1e-6 1 2) SYMBOL nmos -80 160 R0 SYMATTR InstName M3 SYMATTR Value IRL3915 SYMBOL nmos -240 160 R0 SYMATTR InstName M4 SYMATTR Value IRL3915 SYMBOL nmos -400 160 R0 SYMATTR InstName M5 SYMATTR Value IRL3915 SYMBOL nmos -560 160 R0 SYMATTR InstName M6 SYMATTR Value IRL3915 SYMBOL nmos -720 160 R0 SYMATTR InstName M7 SYMATTR Value IRL3915 SYMBOL nmos -880 160 R0 SYMATTR InstName M8 SYMATTR Value IRL3915 SYMBOL nmos -1040 160 R0 SYMATTR InstName M9 SYMATTR Value IRL3915 SYMBOL nmos -1200 160 R0 SYMATTR InstName M10 SYMATTR Value IRL3915 SYMBOL res -1168 32 R0 SYMATTR InstName R10 SYMATTR Value 2.4 SYMBOL voltage -80 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value PULSE(0 5 .2 1e-6 1e-6 1 2) SYMBOL voltage -240 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V5 SYMATTR Value PULSE(0 5 .3 1e-6 1e-6 1 2) SYMBOL voltage -400 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V6 SYMATTR Value PULSE(0 5 .4 1e-6 1e-6 1 2) SYMBOL voltage -560 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V7 SYMATTR Value PULSE(0 5 .5 1e-6 1e-6 1 2) SYMBOL voltage -720 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V8 SYMATTR Value PULSE(0 5 .6 1e-6 1e-6 1 2) SYMBOL voltage -880 288 R0 WINDOW 0 -42 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V9 SYMATTR Value PULSE(0 5 .7 1e-6 1e-6 1 2) SYMBOL voltage -1040 288 R0 WINDOW 0 -57 -3 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V10 SYMATTR Value PULSE(0 5 .8 1e-6 1e-6 1 2) SYMBOL voltage -1200 288 R0 WINDOW 0 -57 1 Left 0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V11 SYMATTR Value PULSE(0 5 .9 1e-6 1e-6 1 2) SYMBOL res -1008 32 R0 SYMATTR InstName R1 SYMATTR Value 2.4 SYMBOL res -848 32 R0 SYMATTR InstName R2 SYMATTR Value 2.4 SYMBOL res -688 32 R0 SYMATTR InstName R3 SYMATTR Value 2.4 SYMBOL res -528 32 R0 SYMATTR InstName R4 SYMATTR Value 2.4 SYMBOL res -368 32 R0 SYMATTR InstName R5 SYMATTR Value 2.4 SYMBOL res -208 32 R0 SYMATTR InstName R6 SYMATTR Value 2.4 SYMBOL res -48 32 R0 SYMATTR InstName R7 SYMATTR Value 2.4 SYMBOL res 112 32 R0 SYMATTR InstName R8 SYMATTR Value 2.4 SYMBOL res 272 32 R0 SYMATTR InstName R9 SYMATTR Value 2.4 TEXT -1186 450 Left 0 !.tran 2 JF
From: ehsjr on 2 May 2010 03:29 Chris W wrote: > ehsjr wrote: > >> >> To Chris W: >> You dismissed it as useless for your purpose because it isn't >> adjustable. But the point was made that you could use the >> resistive element in the tester as the load - don't dismiss >> that idea out of hand, at least until you have investigated. >> > > The fact that it isn't variable wasn't the only thing that led me to > dismiss it. It is designed to draw 100 amps which is more than I want > to draw. So the load it uses has far less resistance than what I need. > I can't think of anyway to increase the resistance of it so I am at a > loss as to how I could use it to make something that varies from as low > as 3 to 5 to as much as 50 amps. > > A bank 50 watt resistors, a large aluminum heat sink and a fan still > seem like the easiest way to go. I just don't like having to switch > various ones in and out to adjust the load but it is certainly doable > just a less than perfect interface. > > If I am missing something on how I could use the resistor in that thing, > please enlighten me. This is a long post. If you understand the guts of it, you'll understand the warning at the end. 2 in series gives you .28 ohms and a 50 watt load. $40 dollars, no heat sink needed (for brief tests), already assembled and less than the $90 dollars you'd spend for 30 resistors at $3.00 each. HF also has a 50 amp unit which could be used, I think. (Not certain about that one) For variability, you'd still need a bunch of power transistors in parallel, but with individual .5 ohm 5 watt emitter resistors. Something like this: + ---+-------------+---ELEMENT---+---+--}}---+ | | | | | P /c | | | 0<--[330R]--| | | | T \e | | | | | /c /c /c | +-----------|---|---}}--| | \e \e \e | | | | | [R] [R] [R] | | | | Gnd -+--------------------------+---+--}}---+ 2N3055's were mentioned in the thread, so we'll discuss that. Say you use 10 2N3055's. At max, each would provide 5 amps to draw 50 amps through the element, but the voltage drop across the transistor would be small, around 1 volt. So the transistors would each dissipate about 5 watts, the emitter resistors would each dissipate about 2.5 watts, while the element dissipated the other ~650 watts. The element could be the 2 series Harbor Freight testers, wirewound resistors, nicrome wire, whatever. For 50A you'll need about 4 amps to drive the transistors' base circuit (all 10 2N3055 bases connected together), so a total of 11 2N3055 power transistors. And they will need to be properly heatsinked. Note that the driver transistor - the left hand 2N3055 - does not have an emitter resistor. Its emitter is connected to drive the bases of the other 2N3055's. You can drive the base of the driver transistor through a 330 ohm resistor connected to the wiper of a 500 ohm 1watt pot, connected across the supply, or circuit of your choice that can provide up to 40 mA to the base of the driver transistor. You need that or similar power transistor circuit to get the 1 potentiometer variability you wanted. Note that at lower current, heat in the transistors will increase. For example, say you want to test at ten amps. Each load transistor will supply 1 amp. The voltage drop in the element will be 2.8 volts, and the drop in the emitter resistors will be ~.5 volts, leaving roughly 10.7 volts across each transistor. Ignoring the driver transistor, that's about 10.7 watts in each of the 10 2N3055 load transistors or a total of ~112 watts to get rid of. The element would dissipate only ~28 watts in that case. The point here is that there will be a lot of heat in the transistors. Your design must take that into account. Also, 2.5 watts (worst case) in each emitter resistor is nothing to sneeze at - they'll be plenty hot when you run the thing at max. Touch one, and you'll get burned. So, you need a big heatsink, forced air cooling (or better) and a cage to prevent accidently touching any of the hot components. Your mention (below) of a 2 or 3 hour test at reduced current implies unattended testing. That calls for more than the simple circuit described so far. Already shown is the change in the location of maximum heat. At max, the majority of the heat is generated in the element while at minimum the majority of the heat is generated in the transistor circuit. Any "non-brief" testing implies unattended testing, and means you need to add some form of safety to the design, because no one's there to keep things under control. For example, say the driver transistor shorts out during the long term test. What was supposed to be a 10 amp (for example) load suddenly becomes a 50 amp load. That's not good news for the "home team" and in fact could burn down the home. That's the primary reason I recommended testing using the HF testing device as designed to be used in my initial post. You're talking a 50 amp current. That is not to be trifled with. What is simple at low current becomes far less trivial at high currents. Aside from destroying the circuit itself and/or burning down the house, an unattended high current load test can destroy the battery or supply being tested, if things go awry or if you overlook something. Example: you set up your load to draw 10 amps from a battery. Something happens, and you don't get back to the thing in 2-3 hours. You could permanently damage the battery that way by discharging too far. That means you need an automatic shut off designed into the load circuit. Obviously, you need some form of thermal shut down if it gets too hot. And, what happens if one transistor shorts or opens? One opening could cause a cascade effect where others open or emitter resistors die. Nothing in the simple circuit is protected. Ed > > Chris W > > BTW I very well may use it to do a full drain test of a battery but > probably not at the full 50 amps so it could be under load for a long (2 > or 3 hours) time. > > > > >> There have been a number of posts addressing resistive elements >> of one sort or another - water tank heaters, hair dryers, >> toasters, headlights. Every one of the ideas mentioned have >> one problem or another associated with them, as well as benefits. >> For example, nichrome wire from toasters or hair dryers requires >> building a safe housing and determining the right means to connect >> to and the correct lengths of nichrome. On the plus side, nichrome >> makes a good element and you can't beat the price of a discarded >> toaster or hair dryer. Headlights are relatively expensive and >> relatively large and require building a mounting panel - but with >> reasonable care they can be used for long periods without overheating. >> The only one that would give you a load in a reasonable size already >> physically mounted in a safe configuration is the Harbor Freight >> tester. Sparky mentioned the problem with it. See my reply >> to him. Don't get me wrong - I am not pushing the tester as the >> best possible source for a resistive element. It is just one >> option. >>
From: John Fields on 2 May 2010 10:25 >On Sat, 01 May 2010 08:07:41 -0500, John Fields ><jfields(a)austininstruments.com> wrote: >>and the finished circuit will look like this, on the ends, with eight >>identical stages in between. >> >> +-----+ >> +-----------------------|+12 | >> | R1 Q1 | | >> +--[2R4]--D S------+--|GND | >> | G | +-----+ >> | | | DUT >>S1>---|-----------+--[1k]--+ >> | | >> . . >> . . >> . . >> | | >> | | >> | | >> | R10 Q10 | >> +--[2R4]--D S------+ >> G | >> | | >>S10>--------------+--[1k]--+ >> >>The advantages? >> >>If you use logic level MOSFETS with an Rds(on) of 20mV at 5A Id, that's >>a dissipation of 100 milliwatts per device, which means _no_ heat sink >>at all. >> >>Plus, you'll be able to drive the gates with 5V CMOS if you want/need >>to. --- Sunday fun; measure the current out of the 12V source: Version 4 SHEET 1 880 1428 WIRE -1888 512 -2160 512 WIRE -1648 512 -1888 512 WIRE -1408 512 -1648 512 WIRE -1168 512 -1408 512 WIRE -928 512 -1168 512 WIRE 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656 R0 SYMATTR InstName M1 SYMATTR Value IRL3915 SYMBOL nmos -1696 656 R0 SYMATTR InstName M2 SYMATTR Value IRL3915 SYMBOL nmos -1456 656 R0 SYMATTR InstName M3 SYMATTR Value IRL3915 SYMBOL nmos -1216 656 R0 SYMATTR InstName M4 SYMATTR Value IRL3915 SYMBOL nmos -976 656 R0 SYMATTR InstName M5 SYMATTR Value IRL3915 SYMBOL nmos -736 656 R0 SYMATTR InstName M6 SYMATTR Value IRL3915 SYMBOL nmos -496 656 R0 SYMATTR InstName M7 SYMATTR Value IRL3915 SYMBOL nmos -256 656 R0 SYMATTR InstName M8 SYMATTR Value IRL3915 SYMBOL nmos -16 656 R0 SYMATTR InstName M9 SYMATTR Value IRL3915 SYMBOL nmos 224 656 R0 SYMATTR InstName M10 SYMATTR Value IRL3915 SYMBOL res 256 528 R0 SYMATTR InstName R10 SYMATTR Value 2.4 SYMBOL res -1904 528 R0 SYMATTR InstName R1 SYMATTR Value 2.4 SYMBOL res -1664 528 R0 SYMATTR InstName R2 SYMATTR Value 2.4 SYMBOL res -1424 528 R0 SYMATTR InstName R3 SYMATTR Value 2.4 SYMBOL res -1184 528 R0 SYMATTR InstName R4 SYMATTR Value 2.4 SYMBOL res -944 528 R0 SYMATTR InstName R5 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trise=30n vhigh=5 SYMBOL Digital\\dflop -1088 928 R0 SYMATTR InstName A5 SYMATTR SpiceLine Td=10n tripdt=10n trise=30n vhigh=5 SYMBOL Digital\\dflop -848 928 R0 SYMATTR InstName A6 SYMATTR SpiceLine Td=10n tripdt=10n trise=30n vhigh=5 SYMBOL Digital\\dflop -608 928 R0 SYMATTR InstName A7 SYMATTR SpiceLine Td=10n tripdt=10n trise=30n vhigh=5 SYMBOL Digital\\dflop -368 928 R0 SYMATTR InstName A8 SYMATTR SpiceLine Td=10n tripdt=10n trise=30n vhigh=5 SYMBOL Digital\\dflop -128 928 R0 SYMATTR InstName A9 SYMATTR SpiceLine Td=10n tripdt=10n trise=30n vhigh=5 SYMBOL Digital\\dflop 112 928 R0 SYMATTR InstName A10 SYMATTR SpiceLine Td=10n tripdt=10n trise=30n vhigh=5 TEXT -2248 1360 Left 0 !.tran 0 42 .001 uic JF
From: John Fields on 2 May 2010 11:51 On Fri, 30 Apr 2010 22:33:07 -0400, "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: > >Chris W wrote: >> >> ehsjr wrote: >> > >> > To Chris W: >> > You dismissed it as useless for your purpose because it isn't >> > adjustable. But the point was made that you could use the >> > resistive element in the tester as the load - don't dismiss >> > that idea out of hand, at least until you have investigated. >> > >> >> The fact that it isn't variable wasn't the only thing that led me to >> dismiss it. It is designed to draw 100 amps which is more than I want >> to draw. So the load it uses has far less resistance than what I need. >> I can't think of anyway to increase the resistance of it so I am at a >> loss as to how I could use it to make something that varies from as low >> as 3 to 5 to as much as 50 amps. >> >> A bank 50 watt resistors, a large aluminum heat sink and a fan still >> seem like the easiest way to go. I just don't like having to switch >> various ones in and out to adjust the load but it is certainly doable >> just a less than perfect interface. >> >> If I am missing something on how I could use the resistor in that thing, >> please enlighten me. >> >> Chris W >> >> BTW I very well may use it to do a full drain test of a battery but >> probably not at the full 50 amps so it could be under load for a long (2 >> or 3 hours) time. > > > >Take a look at: > ><http://www.harborfreight.com/500-amp-carbon-pile-load-tester-91129.html> > > >500 Amp Carbon Pile Load Tester > >Item # 91129 > >Tests 12 volt batteries, alternators, regulators and starters by putting >a load on the system to simulate working conditions. > > * Adjustable load from 0 to 500 amps > * Color-coded temperature compensation pass/fail chart > * Color-coded separate volt and amp meters > * Heavy duty 4 gauge solid copper wire > >Overall dimensions: 10-1/2'' W x 5'' D x 10-1/4'' H >Weight: 8.7 lbs. > >$69.99 --- Caveat... From the manual, at: http://manuals.harborfreight.com/manuals/91000-91999/91129.pdf "15 seconds per test with 1 minute cooldown; 3 tests in 5 minutes." JF
From: Michael A. Terrell on 2 May 2010 15:41
John Fields wrote: > > On Fri, 30 Apr 2010 22:33:07 -0400, "Michael A. Terrell" > <mike.terrell(a)earthlink.net> wrote: > > > > >Chris W wrote: > >> > >> ehsjr wrote: > >> > > >> > To Chris W: > >> > You dismissed it as useless for your purpose because it isn't > >> > adjustable. But the point was made that you could use the > >> > resistive element in the tester as the load - don't dismiss > >> > that idea out of hand, at least until you have investigated. > >> > > >> > >> The fact that it isn't variable wasn't the only thing that led me to > >> dismiss it. It is designed to draw 100 amps which is more than I want > >> to draw. So the load it uses has far less resistance than what I need. > >> I can't think of anyway to increase the resistance of it so I am at a > >> loss as to how I could use it to make something that varies from as low > >> as 3 to 5 to as much as 50 amps. > >> > >> A bank 50 watt resistors, a large aluminum heat sink and a fan still > >> seem like the easiest way to go. I just don't like having to switch > >> various ones in and out to adjust the load but it is certainly doable > >> just a less than perfect interface. > >> > >> If I am missing something on how I could use the resistor in that thing, > >> please enlighten me. > >> > >> Chris W > >> > >> BTW I very well may use it to do a full drain test of a battery but > >> probably not at the full 50 amps so it could be under load for a long (2 > >> or 3 hours) time. > > > > > > > >Take a look at: > > > ><http://www.harborfreight.com/500-amp-carbon-pile-load-tester-91129.html> > > > > > >500 Amp Carbon Pile Load Tester > > > >Item # 91129 > > > >Tests 12 volt batteries, alternators, regulators and starters by putting > >a load on the system to simulate working conditions. > > > > * Adjustable load from 0 to 500 amps > > * Color-coded temperature compensation pass/fail chart > > * Color-coded separate volt and amp meters > > * Heavy duty 4 gauge solid copper wire > > > >Overall dimensions: 10-1/2'' W x 5'' D x 10-1/4'' H > >Weight: 8.7 lbs. > > > >$69.99 > > --- > Caveat... > > From the manual, at: > > http://manuals.harborfreight.com/manuals/91000-91999/91129.pdf > > "15 seconds per test with 1 minute cooldown; 3 tests in 5 minutes." I was thinking of using it in parallel with fixed resistors to trim the load current. I've used the parallel 2N3055 transistors for a variable load before. There was an article in a ham radio magazine years ago, "Power Supply Checker Outer" or something in that vein that had a simple design. At that time the surplus marked was flooded with heatsinks with three to five house numbered 2N3055 transistors for a couple dollars. The magazine was most likely '73', or 'Ham Radio' and in the late '70s or early '80s. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |